1. Field of the Invention
The present invention relates to a constant voltage circuit including an overcurrent protection circuit, a system power device including multiple constant voltage circuits, and a method of controlling the system power device and more particularly to an overcurrent protection circuit for protecting a semiconductor IC in which constant voltage circuits are integrated from overcurrent, high temperature, etc.
2 Discussion of the Background
When constant voltage circuits using series regulators are integrated in an IC, it is typical to attach an output transistor outside the IC since the output transistor consumes great power. However, in the case of when output current is relatively small, for example, in the magnitude of a couple of hundreds mA, output transistors tend to be integrated in the same chip with other circuits for size reduction. Especially, in the case of when a number of series regulators are integrated in one chip, i.e., a system power chip, it is highly effective to build in the system power chip in an output transistor. As a protection device for a constant voltage circuit using a series regulator, an overcurrent protection circuit is typically used to prevent an overcurrent, which is an output current greater than a limit.
FIG. 14 is a circuit diagram illustrating a typical example of a constant voltage circuit having an overcurrent protection circuit.
In FIG. 14, a constant voltage circuit 100 includes a reference voltage generating circuit 101 for generating a reference voltage Vref, an error amplifying circuit A101, an output transistor M101, resistances R101 and R102 for detecting an output current, an output current restriction circuit 102 for restricting the output current from the output transistor M101 and a short circuit current restriction circuit 103 for restricting a short circuit current, which is an output current iout of when an output terminal OUT short-circuits. Since the currents flowing in the resistance R101 and R102 are small and ignorable, the output current from the output transistor M101 is treated to be equal to the output current iout.
Since the drain current of an NMS transistor M105 is the same as the drain current of a PMOS transistor M102, the drain current of an NMOS transistor M106 is a current in proportion to the output current from the transistor M101.
The drain current of the NMOS transistor M106 flows in a resistance R103. Therefore, the voltage drop of the resistance R103 increases as the output current iout increases. When the voltage drop surpasses the threshold voltage of a PMOS transistor M103, the PMOS transistor M103 is turned on and reduces the decrease of the gate voltage of the output transistor M101, thereby restricting the output current iout.
The short circuit restriction circuit 103 includes an operating amplifier circuit A102, PMOS transistors M111 and M112 and a resistance R104.
When the output current restriction circuit 102 starts operating, the output voltage Vout decreases, and a voltage Va at the connection of the resistances R101 and R102 is equal to the voltage drop of the resistance R103, the output voltage from the operating amplifying circuit A102 decreases, resulting in the decrease of the gate voltage of the PMOS transistor M112 Thereby, the PMOS transistor M112 is turned on and the decrease in the gate voltage of the output transistor M101 is restricted. But, there is a difference between both circuits 102 and the operating amplifying circuit A102. That is, in the operating amplifying circuit A102, a voltage Va, which is compared with the voltage drop of the resistance R104, is in proportion to the output voltage Vout so that the current restriction function works to a relatively small output current as the output voltage Vout decreases. Therefore, the output current iout decreases as the output voltage Vout decreases. The input circuit of the operating amplifying circuit A102 has an offset voltage in order that the short circuit current is not 0 A during short circuit. Namely, a short circuit current flows from the output terminal OUT even during short circuit.
In addition, as atypical example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) H04-184606 describes a constant voltage circuit having an overheat protection circuit. The output voltage of the constant voltage circuit is reduced by the output of the overheat protection circuit when the temperature thereof surpasses a limit. JOP 2002-312044 describes a constant voltage circuit which outputs a signal indicating overheat to the central processing unit (CPU) when the output current therefrom and the temperature exceed respective limits.
FIG. 15 is a block chart illustrating a usage example of the constant voltage circuit illustrated in FIG. 14.
In FIG. 15, the constant voltage circuit 100 supplies a voltage to a load 110, which is operated and controlled by a control device 111. When the load 110 is a memory and the control device 111 is a CPU, the memory does not operate when the overcurrent protection circuit of the constant voltage circuit 100 operates and thereby the short circuit current is supplied to the memory 110. However, the CPU 111 does not have a device to acquire information about the state of the memory 100, which may be a drawback because the memory 110 not in activation can freeze the CPU 111.